Amplifiers are devices that accept a varying input signal and produce an output signal that varies in the same way as the input, but with larger amplitude. The input and output signals may consist of a current, a voltage, a mechanical motion, or any other signal. An electronic amplifier is a device for increasing the power of a signal. It does this by taking power from a power supply and shaping the output to match the input signal. This process invariably introduces some noise and distortion into the signal, and the process is not completely efficient. Amplifiers always produce some waste as heat.
Different designs of amplifiers are used for different types of applications and signals. Amplifiers broadly fall into three categories: small signal amplifiers, low frequency power amplifiers, and RF power amplifiers. The most common types of amplifiers are electronic and have transistors or electron tubes as their principal components. Electronic amplifiers are widely used in consumer electronic devices, such as in radio and television transmitters and receivers, as well as audio and stereo systems.
Amplifiers in their simplest form are built around a single transistor. In one type of single-transistor amplifier, known as a common-emitter circuit, a varying input voltage is fed to the base of the transistor, and the output appears at the transistor's collector; the ratio of the output voltage to the input voltage is called the voltage gain. For many purposes a single transistor does not provide sufficient gain, or amplification. In a cascade, or multistage, amplifier, the output of the first amplifying device (transistor) is fed as input to the second amplifying device, whose output is fed as input to the third, and so on until an adequate signal amplification has been achieved. In a device such as a radio receiver, several amplifiers boost a weak input signal until it is powerful enough to drive a speaker. Usually, multistage amplifiers are not made of discrete components, but are built as integrated circuits. Another less common group of electronic amplifiers use magnetic devices as their principal components.
For use with automobile stereo systems, audio amplifiers typically have push-pull outputs, each leg of which is powered directly by a 12-volt car electrical system, which is actually 14.4 volts at full charge. This arrangement, allows each push-pull leg to output about 5 volts root mean square (“RMS”) and summing to 10 volts RMS across the load. The load is “bridged” across the two amplifier stages, hence called a “bridge-tied-load” (BTL). With this type of push-pull amplifier, the 5 volts RMS output voltage is doubled to 10 volts RMS output voltage and the need for a power center-tap load-return is eliminated.
With this typical push-pull amplifier, if the speaker impedance Z=1 ohm, the power equation, P=V2/Z, indicates that 100 watts of audio power is available to the speaker, which in an automobile stereo system is typically a sub-woofer optimized for low frequencies. In some applications more than 100 watts are needed, particularly when the speaker enclosure must be unusually small, as the exciting electrical power required goes inversely with box volume.
This initially appears to be a trivial problem as numerous high-frequency switching techniques utilizing transformers or charge-pumps are commonly employed for increasing supply voltage to amplifiers. However, for equipment supplied in new automobiles, the electromagnetic interference (EMI) generated by such supplies is considered unacceptable. Thus, with an automobile stereo system, one must do the best possible with the available 12 volts of voltage.
In most cases higher amplifier power has been produced by reducing speaker impedance to increase the amplifier output current. In some cases a dual voice coil can be driven by two separate BTL systems for an increase in power. With the peak current in the 1 ohm BTL system exampled above approaching 15 amperes, an attempt to obtain triple power by reducing the load impedance to 0.33 ohm, will result in approaching 45 amperes peak, whether in one voice coil, or summed for a dual-coil arrangement. Parasitic i2r losses are a serious problem for high-power approaches relying exclusively on current-boosting.
Accordingly, there is a need for systems and methods for increasing amplifier power in an efficient manner.